Recent advances in digital cinematic sound provided by the Dolby® Atmos™ system of products have introduced a large increase in the number of independent audio signals used to drive loudspeakers, a large increase in the number of audio tracks carrying aural content that are processed to generate these audio signals, and an increase in the sample bit length and sample rate of digital audio data used to represent these audio signals.
The first generation of Dolby Atmos supports up to 128 discrete audio tracks to generate audio signals that drive up to sixty-four independent channels of loudspeakers. A Dolby Atmos system can provide each loudspeaker channel with its own unique signal, thereby allowing the use of loudspeakers in new positions such as on an auditorium ceiling.
An audio track may carry aural content for an audio object that represents a sound source at some specified spatial position, which is expressed relative to some reference location such as the center of a cinematic projection screen or any other point that may be desired. Any type of acoustic or content-related information such as perceived acoustic size may also be included. The position conveyed by the audio track may change if the sound source is intended to move relative to the reference location. An audio track may also carry aural content for a conventional “fixed channel” that corresponds to one or more loudspeakers at a specified fixed position relative to the reference location.
During playback, the audio tracks are processed to generate the audio signals that are appropriate for driving the loudspeakers that are present for a particular installation. The intent of these advances is to improve the listening experience of cinema viewers by immersing them in an enveloping sound field that is perceived as being as realistic as possible given limits imposed by the number and placement of loudspeakers.
As a simple example for aural content with a position at the center of a viewing screen, a signal is generated for a center-channel loudspeaker if an installation has a center-channel speaker. If an installation has only left- and right-channel loudspeakers, signals are generated for both of these channels so that the aural content is panned to the center position. The process becomes much more complex as the number of audio tracks and loudspeaker locations increase. Furthermore, the computational resources needed to process audio tracks for audio objects is typically much greater than those required to process audio tracks for fixed channels. As a result, a considerable amount of computational resources are needed to process or “render” a Dolby Atmos soundtrack.
The cost of such a processor can be justified for playback systems intended for presentation to audiences but it is much harder to justify for systems like those used for mastering, which require presentation of the audio so that video and audio content can be synchronized and assembled into packages for distribution.